A person having a mass of 100 kilograms who climbs a 3-meter-high ladder in 5 seconds is doing work at a rate of about 600 watts. Mass times acceleration due to gravity times height divided by the time it takes to lift the object to the given height gives the rate of doing work or power.[notes 1]

A laborer over the course of an 8-hour day can sustain an average output of about 75 watts; higher power levels can be achieved for short intervals and by athletes.[2]

The femtowatt is equal to one quadrillionth (10−15) of a watt. Technologically important powers that are measured in femtowatts are typically found in reference(s) to radio and radar receivers. For example, meaningful FM tuner performance figures for sensitivity, quieting and signal-to-noise require that the RF energy applied to the antenna input be specified. These input levels are often stated in dBf (decibels referenced to 1 femtowatt). This is 0.2739 microvolt across a 75-ohm load or 0.5477 microvolt across a 300-ohm load; the specification takes into account the RF input impedance of the tuner.

The picowatt is equal to one trillionth (10−12) of a watt. Technologically important powers that are measured in picowatts are typically used in reference to radio and radar receivers, acoustics and in the science of radio astronomy.

The microwatt is equal to one millionth (10−6) of a watt. Important powers that are measured in microwatts are typically stated in medical instrumentation systems such as the EEG and the ECG, in a wide variety of scientific and engineering instruments and also in reference to radio and radar receivers. Compact solar cells for devices such as calculators and watches are typically measured in microwatts.[3]

The milliwatt is equal to one thousandth (10−3) of a watt. A typical laser pointer outputs about five milliwatts of light power, whereas a typical hearing aid for people uses less than one milliwatt.[4]Audio signals and other electronic signal levels are often measured in dBm, referenced to one milliwatt.

The kilowatt is equal to one thousand (103) watts. This unit is typically used to express the output power of engines and the power of electric motors, tools, machines, and heaters. It is also a common unit used to express the electromagnetic power output of broadcast radio and television transmitters.

One kilowatt is approximately equal to 1.34 horsepower. A small electric heater with one heating element can use 1.0 kilowatt. The average electric power consumption of a household in the United States is about one kilowatt.[notes 2][5]

Also, kilowatts of light power can be measured in the output pulses of some lasers.

A surface area of one square meter on Earth receives typically about one kilowatt of sunlight from the sun (the solar irradiance) (on a clear day at mid day, close to the equator).[6]

The megawatt is equal to one million (106) watts. Many events or machines produce or sustain the conversion of energy on this scale, including large electric motors; large warships such as aircraft carriers, cruisers, and submarines; large server farms or data centers; and some scientific research equipment, such as supercolliders, and the output pulses of very large lasers. A large residential or commercial building may use several megawatts in electric power and heat. On railways, modern high-powered electric locomotives typically have a peak power output of 5 or 6 MW, although some produce much more. The Eurostar, for example, uses more than 12 MW, while heavy diesel-electriclocomotives typically produce/use 3 to 5 MW. U.S. nuclear power plants have net summer capacities between about 500 and 1300 MW.[7]

The gigawatt is equal to one billion (109) watts or 1 gigawatt = 1000 megawatts. This unit is often used for large power plants or power grids. For example, by the end of 2010 power shortages in China's Shanxi province were expected to increase to 5–6 GW[8] and the installed capacity of wind power in Germany was 25.8 GW.[9] The largest unit (out of four) of the Belgian Doel Nuclear Power Station has a peak output of 1.04 GW.[10]HVDC converters have been built with power ratings of up to 2 GW.[11]

The terawatt is equal to one trillion (1012) watts. The total power used by humans worldwide is commonly measured in terawatts (see primary energy). The most powerful lasers from the mid-1960s to the mid-1990s produced power in terawatts, but only for nanosecond time frames. The average lightning strike peaks at 1 terawatt, but these strikes only last for 30 microseconds.

The petawatt is equal to one quadrillion (1015) watts and can be produced by the current generation of lasers for time-scales on the order of picoseconds (10−12 s). One such laser is the Lawrence Livermore's Nova laser, which achieved a power output of 1.25 PW (7015125000000000000♠1.25×1015 W) by a process called chirped pulse amplification. The duration of the pulse was roughly 0.5 ps (6987500000000000000♠5×10−13 s), giving a total energy of 600 J, or enough energy to power a 100 W light bulb for six seconds.[12] Another example is the Laser for Fast Ignition Experiments (LFEX) at the Institute of Laser Engineering (ILE), Osaka University, which achieved a power output of 2 PW (7015200000000000000♠2×1015 W) for a duration of approximately 1 ps.[13][14]

In the electric power industry, megawatt electrical (MWe[16] or MWe[17]) is electric power, while megawatt thermal or thermal megawatt[18] (MWt, MWt, or MWth, MWth) refers to thermal power produced. Other SI prefixes are sometimes used, for example gigawatt electrical (GWe). The International Bureau of Weights and Measures, which maintains the SI-standard, states that further information about a quantity should not be attached to the unit symbol but instead to the quantity symbol (i.e., Pthermal = 270 W rather than P = 270 Wth) and regards these symbols as incorrect and thus non-SI.[19]

Radio stations usually report the power of their transmitters in units of watts, referring to the effective radiated power. It refers to the relative power of the transmission when it is directed towards the horizon for maximum geographic coverage, rather than uniformly broadcast in all directions.

The terms power and energy are frequently confused. Power is the rate at which energy is generated or consumed and hence is measured in units (e.g. watts) that represent energy per unit time.

For example, when a light bulb with a power rating of 100W is turned on for one hour, the energy used is 100 watt hours (W·h), 0.1 kilowatt hour, or 360 kJ. This same amount of energy would light a 40-watt bulb for 2.5 hours, or a 50-watt bulb for 2 hours. A power station would be rated in multiples of watts (for example, the Three Gorges Dam is rated at approximately 22 gigawatts), but its annual energy sales or output would be in multiples of watt hours. Major energy production or consumption is often expressed as terawatt hours for a given period that is often a calendar year or financial year. One terawatt hour is equal to a sustained power of approximately 114 megawatts for a period of one year.

The watt second is a unit of energy, equal to the joule. One kilowatt hour is 3 600 000 watt seconds. The watt second is used, for example, to rate the energy storage of flash lamps used in photography, although the term joule is generally employed.

Terms such as watts per hour (W/h) are often misused when watts would be correct.[20] Watts per hour properly refers to a change of power per hour. Watts per hour might be useful to characterize the ramp-up behavior of power plants. For example, a power plant that changes its power output from 0 MW to 1 MW in 15 minutes has a ramp-up rate of 4 MW/h. Hydroelectric power plants have a very high ramp-up rate, which makes them particularly useful in peak load and emergency situations. The misnomer watts per hour is essentially the derivative of power and should be called rate of watt change per hour.

^The energy in climbing the stairs is given by mgh. Setting m = 100 kg, g = 9.8 m/s2 and h = 3 m gives 2940 J. Dividing this by the time taken (5 s) gives a power of 588 W.

^Average household electric power consumption is 1.19 kW in the US, 0.53 kW in the UK. In India it is 0.13 kW (urban) and 0.03 kW (rural) – computed from GJ figures quoted by Nakagami, Murakoshi and Iwafune.